This invention relates to the field of paint or other protective coating removal from structures such as aircraft.
Protective coatings such as paint are used for a variety of functions on vehicles such as aircraft and on a great number of fixed position objects. In such service, the protective coating provides immunity to oxidation or corrosion, provides thermal insulation and shielding, and of course, is a major tool for appearance enhancement and the provision of camouflage and identification.
During the life of a painted or coating protected object, hereinafter referred to typically as an aircraft, the applied coating often requires removal for a variety of reasons, including replacement of worn and weathered coating materials, and changes in the appearance, camouflage or identification of the object such as might occur in the sale of a used U.S. Air Force aircraft to a friendly foreign nation as part of an arms agreement. The removal of present-day coatings from weapons systems is, however, quite labor intensive and often requires the use of highly activated physical and chemical materials.
Coating removal technology has presently, in short, lagged the development of new polymeric resins in the protective coating art. In the earilier period when alkyd primers, alkyd topcoats and acrylic nitrocellulose topcoats were used as aircraft coating materials, removal was readily accomplished with solvent-based strippers which employed, for example, methylene chloride as a major component. However, as coatings have changed from alkyds and nitrocelluloses to epoxies, polyurethanes, and fluoropolymers, such traditional solvent-based strippers have become inefficient or ineffective in coating removal.
Presently used coatings moreover have a useful life expectancy of 5-7 years as a result of their environmental, erosion, and fluid resistance characteristics. These coatings are therefore capable of enduring beyond the first usage period of a weapon system. Such life is in notable contrast with a functional life of about 2 years for the alkyd and acrylic nitrocellulose coatings previously used. The continued polymerizaton and aging of these newer coatings throughout their life and the resulting increased resistance to chemical stripping materially adds to the difficulty of removing these coatings in the inevitable situation where removal is needed.
The chemical industry has provided improved strippers for use with the presently-used coatings by adding activating agents to the traditional solvent stripper solutions. Commonly used activators include phenols, chlorinated phenols, and amine compounds. However, in addition to being unable to effectively and economically remove epoxy and polyurethane coatings, such compounds are found to pose human health risk and therefore become substances that are regulated by environmental protection agencies and occupational safety and health agencies of the federal and state governments. Phenol-activated strippers are the most effective of these groups, but often require as many as five stripping applications. Such strippers are particularly undesirable in that phenol compounds are not biodegradable and therefore can cause especially difficult environmental and water pollution when used insignificant quantities. The addition of hexavalent chromium compounds to these strippers as a corrosion inhibiting agent further restricts the use of such strippers from an environmental viewpoint.
Chemical paint strippers are also inappropriate for the removal of protective coatings from the new non-metallic organic matrix composite materials now being used in aircraft structures--materials such as an epoxy impregnated fabric of wove graphite filaments. Chemical paint strippers cannot be used for paint removal from such composite materials because of the high risk of the stripper chemically attacking the organic components of the material.
Mechanical paint removal by abrasive blasting is one current alternative to the use of chemical paint stripping. Such abrasive media as crushed corn cobs, glass beads, plastic beads, walnut shells, synthetic diamond dust, garnet particles and dry ice carbon dioxide pellets have been employed in abrasive blasting removal processes. High pressure fluids such as water have also been used in this type of coating removal. All of these techniques have, however, met with such limited success, that a cost-effective and safe arrangement for removing protective coatings, particularly from aircraft structures is yet a pressing need in present-day technology.
The use of plastic beads in abrasive blasting coating removal from aircraft structures and the status of coating removal technology in general is described in a technical report titled "Evaluation of the Effects of a Plastic Bead Paint Removal Process on Properties of Aircraft Structural Materials" published by the Materials Laboratory, Air Force Wright Aeronautical Laboratories, Air Force Systems Command Wright-Patterson Air Force Base, Ohio, 45433, and identified as report number AFWAL-TR-85-4138 dated December 1985. Copies of this report are available from the publishing organization and also from the National Technical Information Service. The contents of the December 1985 AFWAL report is hereby incorporated by reference herein.
As described in the AFWAL December 1985 report, the use of abrasive blasting techniques as an alternate to chemical stripping in metal-skinned and organic matrix composite skinned aircraft raises a number of concerns as to possible undesired side effects of abrasive blasting on an airframe, including the following:
a. Surface roughness and its potential resulting effects on aerodynamic drag; PA1 b. Fatigue properties of the cleaned metal alloys as a result of the induced surface roughness; PA1 c. Removal of protective metal coatings such as aluminum cladding and anodize coatings from aluminum alloys and cadmium plating from steel structure; PA1 d. Effects on the bond strength of aluminum honeycomb and thin skin aluminum metal-to-metal bonded structure. PA1 e. Effects on the physical properties of graphite/epoxy composite materials; PA1 f. Intrusion and consequent effects of the particulate matter on the wear properties of lubricated bearings in the airframe; PA1 g. Thin skin warpage as a result of surface cold working; PA1 h. Effects on fatigue crack growth rate as a result of compressive residual stress on the surface and tensile residual stress in subsurface material; PA1 i. Effects on dye penetrant inspection techniques; and PA1 j. Intrusion of blast partitcles into avionic compartments.
The patent art also discloses the attention of inventors to arrangements for removing paint and other protective coating materials. This attention is evidenced by the patent of J. V. Jones, U.S. Pat. No. 3,623,909, which concerns an electrically heated tool and a method for using the tool in paint removal. Also included in this art are the patents of J. J. Cooney, U.S. Pat. No. 4,380,478, and W. C. Klaiber, U.S. Pat. No. 4,401,476, which each concern arrangements for cleaning paint rollers of the type commonly used for rolling paint onto moderately sized surfaces.
Additionally included in this art is the patent of R. R. Mason, U.S. Pat. No. 4,398,961, which concerns a fuel combustion heated device and method of use thereof for removing old paint. Also included in this art is the patent of W. G. Goerss, U.S. Pat. No. 4,443,271, which concerns an apparatus and method used for cleaning floor grates employing high-pressure water jets.
Further included in this art is the IBM Technical Disclosure Bulletin Vol. 21, No. 7, dated December 1978, entitled "Stripping Procedure for High-Energy and Ion-Bombarded Resists", authored by L. H. Kaplan and S. M. Zimmerman which concerns the removal of resist material layers that have become hard and glossy after high-energy implantation processes and wherein a combination of hot concentrated nitric acid at a temperaure of 80.degree. to 120.degree. C. and ultrasonic agitation are employed. The Kaplan and Zimmerman disclosure bulletin includes at least an inference that the stripping is accomplished in an ultrasonic agitated bath of nitric and phosphoric acids.
It is, of course, well known in the art to employ ultrasonic agitation of a container filled with a solvent or chemical reagent liquid for cleaning purposes. Apparatus of this type has been commercialy available and used, for example, in the cleaning of jewelry and in the cleaning of electronic parts. Ultrasonic energy has also been used for welding and industrial melting attachment arrangements such as in the fabrication of built-up assemblies from plastic component parts.
It may be noted that none of these examples is concerned with the use of ultrasonic energy for the removal of paint or protective coatings from damage-susceptible surfaces such as the exterior of an aircraft.